This application claims the benefits of Japanese Patent Applications P2002-141340, filed on May 16, 2002, and P2003-5601, filed on Jan. 14, 2003, the entireties of which are incorporated by reference.
1. Field of the Invention
The present invention relates to an epitaxial, substrate. The substrate may be used as substrates for photonic and electronic devices, and devices such as a field emitter.
2. Related Art Statement
A film of a group III nitride film has been used as a semiconductor film constituting a semiconductor device such as photonic and electronic devices, and expected, in recent years, as a semiconductor film for a high-speed IC chip used for a mobile phone system. Particularly, a film of a group III nitride containing Al has been expected in an application of a field emitter.
A so-called epitaxial substrate has been used as a substrate having the above nitride film. Such epitaxial substrate has a predetermined substrate and an underlayer film formed on the substrate by epitaxial growth The epitaxial substrate is mounted on a susceptor provided in a reactor, and heated at a predetermined temperature by means of a heating system provided inside or outside of the susceptor. Raw materials for supplying a group III metal and nitrogen and optionally another element are introduced into the reactor with a carrier gas to the epitaxial substrate. A group III nitride film is thus produced using a CVD process to obtain a semiconductor device.
In most cases, the epitaxial substrate, which is produced and removed out from the reactor, is kept in air over several hours or days before the above film forming process is performed. In this case, the properties of the group III nitride film formed on the epitaxial substrate may be changed depending on the ambient conditions in keeping. The properties of the semiconductor device using III nitride film may be also deviated from initially designed values.
Such tendency becomes considerable as the content of Al in the underlayer of the epitaxial substrate is increased. When a group III nitride film is produced on the epitaxial substrate containing the underlayer including Al, the production yield of semiconductor devices using III nitride film on the epitaxial substrate may be thus considerably reduced.
An object of the present invention is, when a group III nitride film is formed on an epitaxial substrate containing an underlayer film containing Al, to reduce the deviation of the properties of the group III nitride film.
Another object of the present invention is to reduce the deviation of the properties of a semiconductor device using the epitaxial substrate and thereby improve the production yield.
The present invention provides an epitaxial substrate, comprising a substrate of a single crystal, an underlayer film provided on the substrate and substantially made of a nitride of a group III element at least including Al, and an oxide film formed on the underlayer. The oxide film has an oxygen content of not lower than 3 atomic percent at the surface and a thickness of not larger than 50 angstrom.
The inventors have studied to provide a solution to the above problems. They finally attained the following discoveries. That is, when an epitaxial substrate is kept in air as described above, the surface may be oxidized to form an oxide film. It is found that the oxide film substantially affects the properties of a group III nitride film after the III nitride film is formed on the epitaxial substrate.
According to the invention, the surface of the epitaxial substrate, specifically the oxide film formed on the underlayer film of the substrate is controlled so that the oxide film satisfies specified properties. The change or deviation of the physical properties of the group III nitride film formed on the epitaxial substrate may be thus successfully reduced. The present invention is based on the discovery.
According to the present invention, as long as the oxide film formed on the underlayer film of the epitaxial substrate satisfies the requirements of the invention, it is possible to reduce the change of the physical properties of a group III nitride film formed on the epitaxial substrate, irrelevant of the state of preservation of the epitaxial substrate. It is thus possible to reduce the change or deviation of properties of a semiconductor device and to improve the production yield thereof.
In a preferred embodiment, the surface roughness Ra of the oxide film may preferable be not larger than 5 angstrom. It is thus possible to effectively reduce the change of physical properties of the III nitride film formed on the epitaxial substrate and to effectively reduce the change of properties of the semiconductor device so that the production yield may be further improved.
Further, the oxygen content at the surface and thickness of the oxide film are measured as follows. That is, the underlayer of a group III nitride is etched in a direction of thickness using Ar ions and the composition is analyzed using ESCA. The thus obtained analytic data in the direction of thickness are used to obtain the oxygen content at the surface and oxide film thickness.
The details for the ESCA analysis are as follows. X-ray having an energy of 1253.6 eV using an Mg target is used. The analysis is performed over a measured area of 1.1 mm xcfx86, a detection angle of 45 degree, and a path energy of 35.75 eV. Oxygen atoms are identified by means of O1s spectra. Pressure during the measurement is 3xc3x9710xe2x88x929 Torr. The etching was performed using Ar+ ions with an accelerating voltage of 3.0 kV in a raster area of 3 mmxc3x973 mm.
The etching rate under the above conditions was proved to be 42 angstrom/min. for SiO2 and xc2xc of that for SiO2 in the case of Al2O3. The sputtering rate of the underlayer is calculated for convenience from the etching rate for Al2O3. The time period required for etching 1 nm is then calculated from the etching rate. The oxygen content at the starting time point of the etching process is deemed as the above oxygen content at the surface. The thickness of the oxide film is calculated based on the etching rate and a time period until the residual surface oxygen disappears. The lower limit for the detection of oxygen is 0.5 atomic percent based on the above process.
Further, in another preferred embodiment, the oxygen content of the underlayer film containing Al under the oxide film is not larger than 1019/cm3 at a depth of 0.2 xcexcm from the surface, which means it is important to reduce oxygen content in the underlayer film under the oxide film as low as possible. The low oxygen content in the underfilm can make the oxide film high quality at the surface of underfilm. As a result, it is possible to reduce the influence of the underlayer film on the change of properties of the group III nitride film and to effectively prevent the change of properties of the semiconductor device so that the production yield may be further improved,
The oxygen content of the underlayer film containing Al is very small as described above and may be below the detection limit of ESCA. The oxygen content is thus measured by means of SIMS.
The details of the SIMS measurement are as follows. A sector magnetic field type-SIMS system, fabricated by CAMECA, is used. 14.5 keV of Cs+ is used as the primary ion and a minus ion is used as the secondary ion. The introduction of oxygen and correction of charge are not performed in the measurement. A gold film is then coated by vapor deposition on the surface of the underlayer film containing Al to prevent the effects of charge up. A cold trap using liquid nitrogen is used to minimize the effects of impurities in atmosphere. Further, the oxygen content at a depth of 0.2 xcexcm from the surface is obtained from a depth profile obtained by etching in the direction of thickness. The value of the oxygen content is decided based on a standard sample for ion implantation. The limit of detection is 7xc3x971016/cm3.
The effects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art.